Curve type-specific sequential rod insertion strategy in combined anterior–posterior surgery for biplanar correction of degenerative lumbar kyphoscoliosis

Technical Summary: Curve Type-Specific Sequential Rod Insertion Strategy in Combined Anterior–Posterior Surgery for Degenerative Lumbar Kyphoscoliosis

Problem Statement
Degenerative lumbar kyphoscoliosis (DLK) presents a unique surgical challenge as a true biplanar deformity, characterized by the coexistence of sagittal and coronal malalignment. While sagittal balance has historically been the primary focus of adult spinal deformity (ASD) correction, coronal malalignment (CM) significantly impacts functional outcomes, pelvic obliquity, and patient satisfaction. Existing techniques for correcting CM, such as asymmetric osteotomies or kickstand rods, were largely developed for coronal-dominant deformities (e.g., degenerative scoliosis) and typically utilize posterior-only approaches. These methods often struggle to address the simultaneous requirement for sagittal realignment in DLK. Furthermore, rigid fractional curves in DLK often resist correction via posterior-only maneuvers, leading to residual deformity that undermines surgical results.

Methodology
This retrospective study analyzed 97 patients with DLK and significant coronal imbalance (Coronal Balance Distance [CBD] > 3 cm) who underwent combined anterior–posterior (AP) surgery between March 2020 and March 2024. The study protocol followed a specific, two-stage surgical strategy:

1. Anterior Mobilization: The procedure began with anterior interbody fusion (OLIF and/or ALIF) to mobilize the rigid fractional curve and horizontalize the L4 and L5 endplates. Bridging osteophytes were removed, and cages were inserted on bilateral cortical rims to alleviate acute fractional angles. Anterior Column Realignment (ACR) was added for rigid segments.
2. Posterior Rod Insertion Sequence: Following anterior release, posterior instrumentation was performed using a curve type-specific sequence based on the Bao classification:
   • Type B Curves (C7 plumb line shifted to the concave side): A contoured rod was first applied to the convex side, followed by a rod-derotation maneuver to realign the C7 plumb line to the midline. The concave-side rod was then applied.
   • Type C Curves (C7 plumb line shifted to the convex side): To avoid aggravating malalignment, the concave-side rod was applied first. Correction was achieved via a translation maneuver, pulling the fractional curve (L4–S1) toward the midline using the mobility gained from the anterior release. Multiaxial reduction screws at L4 and L5 facilitated this translation. The convex-side rod was applied subsequently.
3. Adjunctive Procedures: Posterior Column Osteotomy (PCO) was utilized only when adequate mobility was not restored by the anterior release alone. Pelvic fixation with iliac screws was routine.

Patients were stratified into two groups based on postoperative outcomes: the Coronal Balance (CB) group (CBD < 3 cm) and the Coronal Imbalance (CI) group (CBD ≥ 3 cm). Logistic regression was used to identify factors associated with residual CM.

Key Results
The study demonstrated the feasibility of the proposed strategy for simultaneous biplanar correction:

• Radiographic Outcomes: The cohort showed significant improvement in sagittal and coronal parameters. PI–LL mismatch improved from a mean of 50.7° to 8.8°, and CBD decreased from 4.3 cm to 1.6 cm. Overall, 82.5% (80/97) of patients achieved coronal balance.
• Osteotomy Burden: There was no significant difference in the rate of PCO (71.3% vs. 82.4%) or the number of osteotomy levels between the CB and CI groups, suggesting that correction was driven primarily by anterior mobilization and the rod sequence rather than the extent of posterior osteotomy.
• Predictors of Residual CM: Multivariate analysis identified three independent factors associated with residual coronal malalignment:
  1. Preoperative Type C Curve: This was the strongest predictor, increasing the risk of residual CM by over 15-fold (OR 15.43).
  2. Fewer Anterior Fusion Levels: A lower number of anterior levels was associated with higher risk (OR 0.35 for increased levels).
  3. Greater Preoperative CBD: Larger initial coronal displacement was an independent risk factor (OR 3.50).

Significance and Claims
The authors claim that this study proposes a novel, feasible approach for the simultaneous biplanar correction of DLK by integrating mandatory anterior release with a curve type-specific posterior rod sequence. The significance of this work lies in the interdependence of the two surgical components:

• Anterior Release as a Prerequisite: The study posits that anterior mobilization is essential to render rigid fractional curves amenable to posterior translation maneuvers, particularly for Type C curves, which are otherwise resistant to correction.
• Dependent Sequence: Unlike previous approaches that either relied solely on posterior osteotomy for rigid curves or applied uniform posterior sequences regardless of curve type, this strategy links anterior mobilization directly to a tailored posterior maneuver.
• Feasibility: The strategy successfully corrected both planes of deformity without requiring extensive three-column osteotomies, relying instead on the mobility provided by the anterior approach and the specific rod application sequence.

The paper concludes that while the strategy is effective for most patients, preoperative identification of Type C curves, the magnitude of coronal displacement, and the extent of anterior fusion are critical for predicting and mitigating the risk of residual coronal malalignment. The authors acknowledge limitations, including the lack of a concurrent control group and the retrospective nature of the study, noting that future comparative research is warranted.